An air conditioner requires a particularly high heating capability, and is designed so as to exert the maximum capacity at a time when a heating operation at a low temperature is started. However, normally, a high heating capability is not so required in a long-term operation. The same applies to cooling. Thus, it is not necessary to particularly increase the ability of a motor in a long-term operation. Especially, a load in a long-term operation state is decreasing year by year because of improved thermal insulation of a building. Accordingly, if designing is made so as to satisfy requisitions for the heating capability at a high-load time, an operating efficiency at a time of the long-term operation is deteriorated. In other words, from the viewpoint of energy saving, it is desired to improve particularly a motor efficiency at a low load, to thereby improve an APF (Annual Performance Factor; efficiency of energy consumption efficiency through a year) of the motor.
To improve the motor efficiency at the low load while requiring a heating capability of a high load such as when the heating operation at a low temperature is started, it is conceivable to utilize a boosting of a power source. Others have been also proposed, including a technique of utilizing a magnetic flux weakening control by shifting a phase of a motor current (Japanese Patent No. 3021947), a technique of utilizing a piece of iron which short-circuits a magnetic flux of a magnet (Japanese Patent Application Laid-Open No. 11-275789 (1999)), a technique of utilizing a field control coil (Japanese Patent Application Laid-Open No. 2006-141106), and the like. Also proposed is a technique of using a magnetic flux weakening control and a boosting separately for a heating operation and a cooling operation (Japanese Patent Application Laid-Open No. 2006-313023).